Communications and computer equipment often need to convey digital data between two remotely located positions. As a general rule, digital data are more easily conveyed over shorter distances and/or at slower data transfer rates. However, many techniques are known for transferring digital data over very long distances and at high data transfer rates. These techniques for transferring data over long distances and/or at high transfer rates all suffer a penalty in the form of complicated circuitry and/or complicated data processing. Complications are highly undesirable because they lead to excessive engineering design efforts, reduced reliability, increased installation efforts, increased maintenance efforts, and overall higher costs.
One technique which yields simple and desirable data conveyance implementations involves the use of a gapped clock signal which conveys both frame timing and data or bit timing. The gapped clock signal is transmitted through one communication channel in parallel with another channel which transmits a data signal. The gapped clock indicates when to sample the data signal on the receiving end of the channel so that the data may be successfully recovered. In addition, the gapped clock indicates which data occur at the start of a frame. Consequently, extremely simple transmitter and receiver designs successfully transmit and recover data and partition the recovered data into frames. Unfortunately, conventional gapped clock data transmission schemes are limited to short distances.
Conventional techniques for communicating data at higher data rates and for longer distances are concerned primarily with maximizing the amount of data that can be communicated over a communication medium, such as a fiber optic cable, a coaxial cable, twisted pair cable, RF channel, or the like. Such techniques typically require the multiplexing or mixing of a clock with data. This multiplexing or mixing of clock and data has the desirable attributes of efficiently utilizing the communication media and of preventing clock and data from skewing in time relative to each other at the receiving end.
Unfortunately, the multiplexing or mixing of clock and data has the undesirable attribute of excessively complicated circuitry and/or data processing. At the transmitting end of a communication channel, the clock and data must be mixed together. At the receiving end, the clock must be recovered from the received signal. If a provided clock is not a free-running clock but a gapped clock, then a clock must be regenerated during gap periods. The regeneration of clock signals is well known in the art, and typically uses phase locked loop circuits. However, typical gapped clock signals include gaps of sufficient duration to cause phase locked loop circuits to lose lock or significantly drift. Thus, regenerated clock signals become inaccurate immediately following the gap periods when accuracy is important for conveying frame timing. While other techniques are known and can be devised to interface such signals to conventional high speed and long distance data communication channels, the other techniques tend to become more and more complicated. For example, a simple gapped clock transmission scheme may be converted for transmission over a complicated E1 link through the use of individually configured VME cards for each channel.
In some situations, transmission over long distances requires the use of repeater stations. The use of a single repeater station is an undesirable consequence, and this undesirable consequence is made worse when many repeater stations are needed and when the transmission distance varies from situation to situation. Each transmission situation may require a separate design, and the use of different designs for different situations further complicates the data transmission problem.
Accordingly, a need exists for a simple apparatus and method for conveying a gapped clock signal and associated data over relatively long distances at relatively high speeds.